This paper considers a 3-D thermos-mechanical analysis to investigate the effect of welding direction, tack weld, root gap and welding process on welding deformation in hub flange and pipe joint. Multi-pass welding for a single ‘V’ butt-weld joint geometry is simulated and 3-D weld path analysis is used with realistic weld bead. 3 different welding direction (2-path, 4-path diagonally, 4-path from the bottom) are analyzed and how the presence or absence of tack weld affects deformation was analyzed. Also, 3 different cases for root gaps (2, 3, 4mm) are considered and finite element analysis results are compared. In addition, the effects of the welding process on the hub flange face are determined. From the results, perpendicularity of the hub flange face is found to be strongly dependent on the welding direction and tack welds. On the other hand, root gap has small influence for tilting. The results also showed the feasibility of mitigating and controlling the welding deformation, and optimum selection of welding method was confirmed.